A second suspect, a "very wanted man" believed to be a computer and communications expert, has also been seized by the Pakistani authorities.

The computer engineer was sending coded messages to al-Qaeda suspects, according to one intelligence official.

Ahmed would not reveal whether the information from Ghailani or the computer expert had prompted US Homeland Security Secretary Tom Ridge's warning.

Employees of some of the most famous financial institutions in the US have been urged to report for work despite the "credible" threat from al-Qaeda.

British shadow home secretary David Davis called on Prime Minister Tony Blair to "spell out the exact threat to the UK".

He said: "The British Government should take this risk extremely seriously. We find it very worrying that the Americans seem to be at a much more advanced stage than us in contingency planning and police presence. They also share much more information with their public than our Government.

"Mr Blair needs to spell out the exact threat to the UK so that we are in a clear position as to where we stand. It is astonishing that we are getting more information about the risk to Britain from the Americans than from our own Government.

"The raw truth is that local authorities are under-resourced and Britain does not have one person solely responsible for the job of keeping us safe against terrorists. The sooner we have a Minister for Homeland Security, the better."